(1) Field to which Invention Relates
The invention relates to a method for the optical determination of departures in shape, changes in shape, and changes in position, in the case of which by means of light rays patterns are produced on the object and the patterns are represented by a photoelectronic device.
(2) The Prior Art
The wording used referring to determination of a departure in shape is to be understood to be a comparison between the actual shape of an object and its ideal shape, which is represented by a master object, a model after scale transformation or an ideal object which is determined only by computation or graphically.
By the reference to determining a change in shape and a change in position a comparison of the shape or position of one and the same object with respect to its changes in time is to be understood.
In addition to the departure in shape a departure in position of an object with respect to its ideal condition or state as defined by a master object, model, computed value or graphical representation is possible or it is also possible to make a comparison as regards a simple change in position or change in shape, for example when a turbine blade to be measured is twisted in its angle with respect to the axis of rotation of the turbine. The departure in position is however, from the point of view of the method technology employed here, only a subspecies of a departure in shape.
Determination and comparison, carried out with the highest degree of accuracy, of the shape in position of objects has become highly significant in technology and in what follows the important examples of application will be given:
In the production of complicated technical shapes, as for example turbine blades or blades of ships propellers the departure from the ideal state, both as regards the shape and also the position, must be determined in a true-to-shape examination. Oscillations of components can lead to the components breaking, especially if they occur close to the resonance range. Furthermore, they can cause substantial environmental trouble owing to radiation of sound or they may impair the functional efficiency of whole machines and plant as for example printing machines. In order to control such oscillations or vibrations, a knowledge of their shape and amplitude is necessary; In harmonic analysis it is possible, for example by comparison of two positions of the oscillating component, to obtain the necessary information. The deformation of components under static loading can lead to such components being damaged or destroyed. It is therefore necessary to avoid localised load peaks during design. The possibilities of solutions by calculation are often limited. It is however possible to determine in tests the positions of maximum loading by comparisons in shape between the loaded state and the unloaded state. Faults in materials, as for example flaws in the case of cast metals or separation into layers in the case of motor vehicle tires, can lead to irregular deformations under a pre-established load, which can be determined by a comparison as regards shape of the condition of the object before and on change in for example the ambient pressure or the thermal state of the object or of its static loading in material examination.
The certainly most familiar method of true-to-shape comparison is contact sensing or scanning of the article to be tested along one coordinate, in which respect reference points have to be established. It is possible to obtain the profile or contour in absolute values by the reading given on a clock gauge or digitally and the so established actual value can be compared with the values of a master sample or with calculated values. The requirements as regards working time and the number of personnel are however substantial. Laser technology makes possible a series of contact-free methods for true-to-ahspe comparison. In the case of one of these methods the laser beam is focussed on one point of the surface and the travelling time of the reflected beam is electronically measured in accordance with the radar principle. This method is used for example for the measurement of tire moulds. Holographic methods have as yet not found any acceptance in actual practice owing to their complicated performance. They can only be used to produce digital results with substantial calculating equipment using displacement factors (see Steinbichler et al.: "Quantitative Auswertung von Hologrammen" in Laser-Elektro-Optik No. 5/1973). In the lattice projection method previously proposed, but not as yet published these disadvantages are admittedly substantially circumvented and automatic evaluation is admittedly possible in this case but however it requires a high degree of complexity (German patent application No. 24 10 947.5). Finally, attention is also to be drawn to a non-laser technique derived by Takasaki, which is simple as regards the aspects of complexity and actual performance, but whose accuracy of measurement cannot be considered satisfactory (Takasaki: Moire Topography in "Applied Optics," 1970, pages 1467 to 72).
For haromic analysis so-called accelerational pick-ups or accelerometers are used, which can be arranged at specific points, for measuring vibrations or oscillations. Radiation of sound can be measured with suitable microphones. The detection of fields is based on a stationary condition. The holographic time-average method and double-pulse holography for aperiodic vibrations admittedly represented a substantial advance, however in this case as well the limitations mentioned in the case of holographic methods apply.
For static design optimisation numerous methods of measurement are available. These extend from punctuate determination of deformation with the help of sensors via strain gauges to photoelastic methods. In this context substantial advantages are offered by holographic interferometry. As compared with photoelastic methods, there are possibilities of measurement on the actual object while as compared with the strain gauge method there is the advantage of rapid provision of results. However, there is the disadvantage, apart from the degree of complexity, of the excessive sensitivity of the holographic method. Furthermore, the evaluation of holographic interferograms is problematical, since the total deformation vector makes a contribution to the interference display, while in general however only the vector components are relevant. More especially however digital representation gives rise to substantial difficulties.
In the case of the non-destructive testing of materials methods using ultrasonics, X-rays, thermography, and sound emission stand in the foreground, which are supplemented by the holographic method. Its high sensitivity admittedly has the advantage that the deformation necessary for indicating a fault is far removed from any damage or destruction. However, it involves the disadvantage that much complexity is called for in order to avoid the effect of ambient conditions as for example oscillations in the floor.
Holographic interferometry is at the present time the furthest advanced method in the sector coming into question. The advantage of the pictorial information which can be obtained with it and the very high accuracy of measurement has to be weighed up against the disadvantage of the complicated apparatus and the complex operational steps required and furthermore the necessity of cancelling out and avoiding ambient effects. These disadvantages impair or prevent however the direct use of these methods in industrial production.